Method of making a multiphase hot-rolled steel strip

ABSTRACT

The invention concerns a method for making a multiphase hot-rolled steel strip comprising an ultra-fast cooling operations, which consists in carrying out said ultra-fast cooling operation after controlled slow cooling of the strip on a conventional slow cooling table of the rolling mill. The controlled cooling constitutes a first slow cooling, at the output of the finishing mill, from an end-of-roll temperature to an intermediate temperature of about 750° C. to 500° C.; said first cooling determines the fraction of the first phase (ferrite) in the steel. The ultra-fast cooling (&gt;150° C./s), which solidifies the resulting structure, lowers the temperature of the strip down to a coiling temperature, ranging between about 600° C. and room temperature, at which a second slow cooling is performed which results if the formation of the second phase (bainite or martensite).

This is a national stage of PCT/BE01/00015, filed 29 Jan. 2001 andpublished in French.

TECHNICAL FIELD

The present invention relates to a method for making a multiphasehot-rolled steel strip having improved mechanical properties, inparticular high strength and good ductility. Currently, such strips havea thickness of between 0.7 mm and 10 mm and more often between 2 mm and6 mm.

PRIOR ART

High-strength steels have been known for a long time in the prior artand they have many different uses. In many cases, the mechanicalproperties of these steels result from appropriate thermal treatment,allowing in many cases to avoid having recourse to alloying elements,which are generally expensive.

However, certain applications require hot-rolled steel strips that haveboth high strength and good forming properties. Currently, such acombination of properties is extremely difficult to achieve and moreoveris generally obtained only by means of multiphase steels such as steelswith a ferrite/bainite or ferrite/martensite microstructure or bythree-phase steels. In these steels, the ferrite forms the ductile anddeformable element, while the second phase, bainite or martensite,strengthens the steel. The final mechanical properties of the steel aredirectly affected by the respective proportions of these phases and bythe temperatures at which these are formed.

According to conventional practice, steels with a ferrite/bainite orferrite/martensite microstructure are obtained from a specific chemicalcomposition and by strict control of the cooling conditions during hotrolling. The microstructure and properties of these steels are affectedby the coiling temperature and by the cooling rates to which the steelsare subjected.

On a conventional laminar cooling table, it is not possible to controlthe cooling rate of the hot-rolled strip because the specific deliveryrates of the cooling liquid are fixed. This cooling rate will thereforelargely depend on the speed and thickness of the strip and on externalparameters such as the temperature of the cooling liquid. In particular,it varies over the length of the strip owing to the increase in thespeed of the latter due to the acceleration of the rolling mill betweenthe beginning and the end of a strip. As is known, this acceleration isimposed by the need to maintain a constant end-of-roll temperature forthe entire strip. This results in uncertainty as to the cooling rate ofthe steel, which has repercussions for the microstructure and henceproperties of the strip and may ultimately be translated into costlystrip cropping and degradation.

Moreover, the chemical composition of the steel must be adapted as afunction of the microstructures to be achieved and likewise as afunction of the cooling which might be applied. In these conditions, itis virtually impossible to vary the composition of the steel in aspecific way in order to improve certain mechanical properties, such asfatigue resistance or resistance to ageing, capacity for hole expansion,or indeed suitability for welding or surface quality.

It is furthermore known that it is possible to produce multiphase steelsby a cooling treatment referred to as interrupted-cycle treatment. Ingeneral terms, such treatment initially comprises a first step, in whichthe strip is maintained at a high temperature to ensure partialtransformation of the austenite into ferrite, followed by abrupt coolingintended to solidify the partially transformed microstructure, andfinally a second step, in which the temperature is maintained at a lowerlevel to transform the rest of the austenite into bainite or intomartensite. In conventional strip mills, the cooling tables do nothowever have cooling sections that are powerful enough to ensure abruptcooling of this kind.

In this regard, an ultra-fast cooling method (UFC) is indeed known,applied to a hot-rolled strip immediately after it emerges from thefinishing mill. This ultra-fast cooling is followed by slow cooling,referred to as laminar cooling, on the conventional cooler leading tothe coilers. This method does, of course, allow to obtain steels with ahigh elastic limit, e.g. steels containing dispersoids. However, suchsteels have a lower ductility than that developed by multiphasestructures, preventing them from being used for applications thatrequire one or more forming operations.

PRESENTATION OF THE INVENTION

The present invention aims to propose a method for making a multiphasehot-rolled steel strip which has mechanical properties, in particularstrength and ductility, that are improved compared to theabove-mentioned prior art.

According to the present invention, a method for making a multiphasehot-rolled steel strip, which comprises an ultra-fast cooling operation,is characterised in that said ultra-fast cooling operation is carriedout after slow laminar cooling of the strip on the cooling table andbefore the final coiling of the strip.

In hot-strip mills, the end-of-roll temperature of the strips is equalto or greater than the Ar3 transformation temperature; of course, thistemperature varies as a function of the composition of the steel but itis generally between about 800° C. and 900° C.

According to the invention, the hot-rolled steel strip is subjected, onemerging from the finishing mill, to a first slow cooling operation fromthe end-of-roll temperature to a temperature referred to as theintermediate temperature, between about 750° C. and 500° C., preferablybetween 750° C. and 600° C., then to an ultra-fast cooling operationfrom said intermediate temperature to a temperature referred to as thecoiling temperature, between about 600° C. and room temperature, andfinally to a second slow cooling operation from said coiling temperatureto room temperature.

The first cooling operation preferably takes place on the conventionallaminar cooling table, i.e. with water at a low cooling rate; however,it can also be carried out with air. It thus forms the first step inwhich the strip is maintained at a high temperature, during which theferrite can form in conditions close to equilibrium. The duration ofthis first cooling operation depends on the speed of the strip and onthe cooling rate applied, as a function of the degree of transformationdesired and hence of the intermediate temperature intended. The coolingrate being low in all cases, it is not influenced to any significantextent by the effect of the acceleration of the mill.

The abrupt cooling operation is then preferably carried out by theultra-fast cooling method mentioned above. It may be recalled here thatthis ultra-fast cooling consists in spraying the strip with jets ofwater under a pressure of 4 to 5 bar; this cooling can be regulated interms of cooling rate and temperature by means of the water deliveryrate and the length sprayed. It allows to achieve cooling rates of 5 to10 times greater than conventional laminar cooling tables. Saidultra-fast cooling operation is preferably carried out at a cooling ratesuch that the product of the thickness of the strip in mm and thecooling rate in ° C./s is greater than 600, and preferably greater than800. By way of illustration, the ultra-fast cooling operation mentionedabove is advantageously carried out at a cooling rate greater than 150°C./s on a 4-mm thick strip.

Finally, the second slow cooling operation is carried out immediatelyafter the abrupt cooling operation, i.e. essentially during the coilingof the strip. This cooling operation takes place from the coilingtemperature to a temperature at which there is no more transformation ofthe microstructure, i.e. in practice to room temperature. In the courseof this slow cooling operation, the residual austenite is generallytransformed to form the second phase, bainite or martensite, as afunction of the coiling temperature. However, in certain cases, thistransformation may take place before the slow cooling operation, i.e.during the abrupt cooling operation.

For the practical implementation of the invention, the respectiveproportions of the phases required in the steel are first of alldetermined as a function of the desired properties; the duration of thefirst slow cooling operation and the intermediate temperature leading tothe required fraction of the first phase are deduced therefrom; thecoiling temperature leading to the required second phase is likewisededuced therefrom; finally, said values for duration and temperature areapplied for the respective regulation of the first slow cooling and theultra-fast cooling stages.

EXAMPLES

By way of example, the method according to the invention has beenapplied to a first series of steel grades, the chemical compositions ofwhich are given in Table 1.

TABLE 1 Chemical composition (without precipitation) ChemicalComposition (10⁻³%) Grade C Mn Si Al N Nb Ti 1 144 996 7 32 4 0 1 2 67760 4 31 3 48 30 3 80 1448 122 27 5 32 1

In conventional practice, steel 1 can lead to a dual-phasemicrostructure (ferrite/bainite but not ferrite/martensite). Steel 2will not form a multiphase microstructure owing to the high contents ofniobium and titanium, which cause a very rapid transformation of theaustenite into ferrite and pearlite, thereby counteracting the formationof bainite and/or martensite. Finally, steel 3 allows in principle theformation of a dual-phase microstructure (ferrite/martensite) thanks toits high manganese contents and a carefully chosen thermomechanicalcycle. However, such a transformation is only accomplished withdifficulty on the laminar cooling table and entails a significantreduction in the productivity of the hot-rolling mill.

These three steels were subjected to a treatment cycle according to theinvention, the of which are indicated in Tables 2 and 3 for steels witha ferrite/bainite (Table 2) and a ferrite/martensite or dual-phasemicrostructure (Table 3) respectively. These two tables likewise showthe properties and fractions of the second phase of the steelsconsidered.

TABLE 2 Ferrite/bainite steels Rolling Intermediate Coiling YS TSUniform Total elonga- temper- tempera- tempera- Elastic Breaking elonga-tion T.El Bainite Grade ature ture ture limit load tion (L0 = 50 mm)YS/TS TS * T.El fraction 1 830° C. 715° C. 550° C. 331 467 17 29 0.7113707  45% 2 890° C. 712° C. 600° C. 455 508 15 29 0.90 14466  10% 2890° C. 745° C. 550° C. 456 523 14 26 0.87 13779  25% 3 870° C. 670° C.550° C. 475 549 15 26 0.86 14287 ˜30% 3 870° C. 700° C. 600° C. 480 55612 23 0.86 12501 ˜40% 1 840° C. 715° C. 275° C. 381 585 12 23 0.65 13368 45% 3 870° C. 670° C. 350° C. 515 632 10 20 0.81 12649  60%

This Table 2 shows that it is possible to obtain multiphasemicrostructures with improved properties of strength and ductility fromeach of these three grades of steel. This result is obtained by carefulchoice and adequate control of the intermediate temperature and thecoiling temperature. The choice of coiling temperature allows toregulate the fraction of ferrite transformed and, consequently, also thefraction of the second phase; that of the coiling temperature allows todetermine the nature of this second phase (bainite or martensite). Ifthis coiling temperature is carefully chosen, it can likewise allow theappearance of a third phase. This is the case, in particular, between200° C. and 350° C., where a fraction of martensite may appear within aferrite/bainite microstructure.

TABLE 3 Dual-phase steels Rolling Intermediate Coiling YS TS UniformTotal elonga- temper- tempera- tempera- Elastic Breaking elonga- tionT.El Martensite Grade ature ture ture limit load tion (L0 = 50 mm) YS/TSTS * T.El fraction 2 900° C. 660° C. Room 515 695 11 18 0.74 12453  5% 3870° C. 660° C. Room 430 706 11 18 0.61 12824 45% 2 900° C. 690° C. Room532 711 11 16 0.75 11517 10% 3 870° C. 630° C. Room 450 743 13 19 0.6114119 15-20% 1 830° C. 665° C. Room 459 783 10 15 0.59 11648 17% 3 870°C. 707° C. 100° C. 496 812 11 20 0.61 16240 60% 3 870° C. 707° C. Room507 839 10 16 0.61 13277 60% 1 830° C. 715° C. Room 488 856 10 13 0.5710877 35%

Table 3 shows that ultra-fast cooling of these same steels to a coilingtemperature equal to room temperature leads to the formation ofmartensite and, consequently, to increased strength while preservinggood ductility. The coiling temperature of 100° C. corresponds to slightreheating of the strip after cooling, which does not prejudice itsstrength and even slightly improves its ductility.

In a second example, micro-alloyed steels were likewise subjected to acycle of treatment according to the invention. Their chemicalcompositions are given in table 4.

TABLE 4 Chemical composition (with precipitation) Chemical composition(10⁻³%) Grade C Mn Si Al N Nb Ti 4 80 1000 30 100 5 80 1500 30 100

The cooling schemes are indicated in Tables 5 and 6 for steels with aferrite/bainite microstructure (Table 5) and a ferrite/martensitemicrostructure (Table 6) respectively. Such cooling schemes inaccordance with the invention enable the hardening of the steels byprecipitation of micro-alloying elements (Ti) in the form of carbides.Such precipitation is generally impossible in a conventional multiphasesteel because it requires a first, very slow cooling operation (<20°C./s) at high temperature (>600° C.).

Tables 5 and 6 likewise show the properties of strength and ductilityobtained with these steels.

TABLE 5 Ferrite/bainite steels Rolling Intermediate Coiling YS TSUniform Total elonga- temper- tempera- tempera- Elastic Breaking elonga-tion T.El Bainite Grade ature ture ture limit load tion (L0 = 50 mm)YS/TS TS * T.El fraction 4 640° C. 450° C. 547 606 14 23 0.9 13938 5650° C. 450° C. 650 706 11 21 0.92 14826

TABLE 6 Dual-phase steels Rolling Intermediate Coiling YS TS UniformTotal elonga- temper- tempera- tempera- Elastic Breaking elonga- tionT.El Martensite Grade ature ture ture limit load tion (L0 = 50 mm) YS/TSTS * T.El fraction 4 650° C. Room 539 743 12 21 0.73 15603 5 650° C.Room 601 853 10 17 0.7 14501

The method according to the invention offers several significantadvantages over the prior art.

Firstly, it allows better control over the formation of microstructures,namely the fraction of ferrite, on the one hand, and the fraction andnature of the second phase, on the other hand. The microstructures ofthe two phases are in fact obtained by two totally independent coolingoperations, which enable to manage and regulate the temperatures leadingto the desired microstructures separately.

The first of these two cooling operations is carried out on the laminarcooling table, starting from the end-of-roll temperature. Since thecooling rate is not very high here, it is not very critical and isscarcely influenced by the effect of acceleration of the rolling mill.This operation allows to regulate the percentage of ferrite formed byvarying the cooling conditions, in particular the number of sectionswhich are sprayed, i.e. in fact the duration of cooling, to obtain thedesired intermediate temperature.

The second cooling operation is an abrupt cooling operation, preferablyultra-fast, to the coiling temperature corresponding to the desiredmicrostructure of the second phase, whether this is bainite ormartensite. The effect of this cooling is to solidify the microstructureformed in the course of the first slow cooling operation so as to allowthe transformation to resume at the coiling temperature.

The microstructures being controlled by means of the temperatures of thetreatment cycle, it is consequently possible to obtain differentmechanical properties starting from the same grade of steel. The methodaccording to the invention likewise allows to create multiphasemicrostructures and to give interesting properties to grades of steelthat had not previously been intended for this purpose.

Moreover, the method according to the invention is no longer limited toa limited number of specific chemical compositions to obtain the desiredmicrostructures. Indeed, these microstructures no longer depend on thechemical composition of the steel but are the outcome of numerous waysof combining the slow laminar cooling and the abrupt cooling whichfollows it. It is consequently possible to adapt the chemicalcomposition of steels more easily to improve their mechanicalproperties, such as resistance to fatigue or ageing, suitability forwelding or hole expansion, surface quality or suitability for cutting.It can likewise result in a reduction of the costs of steel productionwhich are linked, for example, to a drop in productivity or tooperations such as the repairing of cracks or descaling.

What is claim is:
 1. A method for making a mutliphase hot-rolled steelstrip comprising the step of carrying out an ultra-fast coolingoperation, which comprises a spraying of the strip with water jets undera pressure of 4 to 5 bar, keep a slow laminar cooling of the strip on acooling table and before a final cooling of the strip, and at such acooling rate that the product of the thickness of the strip in mmmultiplied by ° C./s is greater than
 600. 2. The method according toclaim 1, wherein, on emerging from a finishing mill, the hot-rolledsteel strip is subjected to a first slow cooling operation from anend-of roll temperature to a temperature referred to as an intermediatetemperature, between about 750° C. and 500° C., then to an ultra-fastcooling operation from said intermediate temperature to a temperaturereferred to as a coiling temperature, between about 600° C. and roomtemperature, and finally to a second slow cooling operation from saidcoiling temperature.
 3. The method according to claim 2, wherein theintermediate temperature is between about 750° C. and 600° C.
 4. Themethod according to claim 1, wherein said first slow cooling operationis carried out on a conventional laminar cooling table arrangeddownstream of a finishing mill.
 5. The method according to claim 1,wherein said ultra-fast cooling operation is carried out at such acooling rate that the product of the thickness of the strip in mm andthe cooling rate in ° C./s is greater than
 800. 6. The method accordingto claim 1, wherein the respective proportions of the phases required inthe steel are determined; the duration of a first slow cooling operationand an intermediate temperature leading to the required fraction of afirst phase are deduced therefrom; a coiling temperature leading to arequired second phase is likewise deduced therefrom; and, said valuesfor duration and temperature are applied for the respective regulationof the first slow cooling operation and of the ultra-fast coolingoperation.